Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 72,943	$ 67,811	$ 62,690
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(16,184)	(14,647)	(14,181)
Net unpaid losses and loss expenses, beginning of year		56,759	53,164	48,509
Net losses and loss expenses incurred in respect of losses occurring in Current Year		24,495	22,966	22,124
Prior Year Claims and Claims Adjustment Expense	[2]	(1,153)	(986)	(414)
Total		23,342	21,980	21,710
Net losses and loss expenses paid in Current Year		8,117	7,836	7,782
Net losses and loss expenses paid in Prior Year		12,206	10,048	9,652
Total		20,323	17,884	17,434
Foreign currency revaluation and other		(583)	(501)	379
Net unpaid losses and loss expenses, end of year		59,195	56,759	53,164
Reinsurance recoverable on unpaid losses, end of year	[1]	17,128	16,184	14,647
Gross unpaid losses and loss expenses, end of year		76,323	72,943	67,811
Prior Period Development, net adjustments		$ 277	$ 60	$ 19

[1] Net of valuation allowance for uncollectible reinsurance.

[2] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $277 million, $60 million, and $19 million for 2022, 2021, and 2020, respectively.